MEMS scanning devices find application in a wide variety of electrical, mechanical, and optical systems. A non-exhaustive list of applications includes scanners, displays, projectors, switches, printers, barcode readers, retinal displays, resonators, and sensors. MEMS scanning devices may be driven by, for example, electrostatic actuation, electromagnetic actuation, a combination of electrostatic and electromagnetic actuation, and piezoelectric actuation.
In scanning applications, MEMS devices are typically driven at their resonant frequencies to produce the desired scanning angle and scanning speed. When using a MEMS device as a mirror in a scanning device, the mirror size affects the resulting resonant frequency. If a large mirror size is used, it is difficult to obtain a high resonant frequency. If the mirror size or mass is decreased, the resonant frequency increases.
Various approaches have been used to alter the resonant frequency of MEMS devices. U.S. Pat. No. 6,256,131 describes a MEMS mirror including selectively removable tabs. The resonant frequency is measured and tabs are removed via laser trimming to reduce the mass of the mirror body to increase the resonant frequency to a desired frequency.
U.S. Pat. No. 7,034,370 uses a voltage differential between electrodes to tune the natural frequency of a MEMS structure and thereby increase the manufacturing yield.
U.S. Pat. No. 6,753,639 discloses a MEMS microbeam oscillator which has material added to or decreased from its surface to tune the oscillator. The material is ablated via a laser following measurement of the resonant frequency of the oscillator. Similarly, material may be deposited onto the upper surface of the microbeam oscillator to tune the device.
U.S. Pat. No. 7,187,488 uses laser or ion beam trimming of a MEMS mirror in a sacrificial portion to fine tune the natural frequency of the device. U.S. Patent Application Publication 2010/0002284 describes a method of modulating the resonant frequency of a torsional MEMS device. The resonant frequency of a MEMS device is measured and if it is greater than a standard resonant frequency, a mass increaser is bonded to the back surface of the MEMS device. As shown in the figures, these mass increasers are positioned along the single torsional axis of the MEMS device.
There remains a need in the art for improved techniques for altering the resonant frequency of MEMS devices, particularly for reducing the resonant frequency of MEMS devices.